Accessory abductor hallucis causing entrapment of the posterior tibial nerve.

A young man, known to have had juvenile rheumatoid arthritis since the age of seven, was seen again at the age of 24 when he developed excruciating pain over the medial aspect of his right foot and ankle. The pain, of four days’ duration, was aggravated by any movement of the foot or ankle, by weight-bearing and by pressure. There was no known relieving factor nor any history of injury. On examination, there was slight swelling behind the medial malleolus and the tibialis posterior was in spasm. The posterior tibial nerve was markedly thickened and tender. Percussion of the nerve just behind the malleolus increased the intensity of his symptoms. He had altered sensation along the medial border of the foot and great toe, but no motor involvement. A provisional diagnosis of entrapment of the posterior tibial nerve due to rheumatoid tenosynovitis was made. Electromyography and nerve conduction studies revealed attenuation in amplitude of the sensory action potential from the right posterior tibial nerve (2 iV) as compared with the left (7 zV), though the latency and velocity were normal. The motor component was unaffected. After a four-week trial with non-steroidal anti-inflammatory drugs, the right posterior tibial nerve was surgically decompressed. Under general anaesthesia and a tourniquet, an incision was made about 2.5 cm behind the medial malleolus, commencing 5 cm proximal to its tip and extending to 2.5 cm below it. On incising the subcutaneous tissue, muscle fibres were visible in the proximal part of the wound (Fig. 1) originating from the under surface of the fascia covering the posterior tibial nerve. This fascia was incised to reflect the origin of the muscle posteriorly and expose the thickened nerve. A muscle belly was then seen to pass deep to the nerve (Fig. 2), through the tarsal tunnel and to merge with the main abductor hallucis muscle. It had no separate nerve